This invention relates generally to methods for creating fatigue-resistant and damage-tolerant components more specifically to a method of setting process parameters for a burnishing treatment.
Various metallic, ceramic, and composite components, such as gas turbine engine fan and compressor blades, are susceptible to cracking from fatigue and damage (e.g. from foreign object impacts). This damage reduces the life of the part, requiring repair or replacement. The main objective of burnishing is to impart residual stress onto a surface to obtain material benefits, like fatigue and corrosion resistance and preventing crack formation and propagation. Of these benefits the aerospace industry is most interested in increasing fatigue life stress resistance. It is known to protect components from crack propagation by inducing residual compressive stresses therein. Methods of imparting these stresses include shot peening, laser shock peening (LSP), pinch peening, and low plasticity burnishing (LPB). These methods are typically employed by applying a “patch” of residual compressive stresses over an area to be protected from crack propagation.
A typical burnishing apparatus includes rolling burnishing elements such as cylinders or spheres which are loaded against a workpiece at a selected burnishing pressure by mechanical or hydrostatic means, and traversed across the part surface in a series of strokes or segments. The magnitude of the residual stress is a function of a number of parameters, of which the most influential are the burnishing pressure and the degree of overlap of burnishing strokes. With the high costs of fatigue testing, the initial selection of these parameters can prove expensive given the broad range of burnishing pressures and degrees of overlap.
In the prior art, initial pressure and overlap selection is performed either arbitrarily or through trial and error. A trial and error approach is not only expensive but time consuming.
Furthermore, using parameters derived for a particular application may not have the same results for another application. For example, burnishing two thin plates of the same material under the same conditions but with different cross-sectional thickness will result in different degrees of overlap up to a critical thickness, and therefore will behave differently in fatigue testing. The critical thickness is the thickness for a given material at which the degree of overlap will remain constant at or above this value, if all other parameters are held constant.